"Hello Ajax"! How to Do the Ajax Thing with Oracle JDeveloper

Get an introduction to Ajax programming by creating a simple "Hello World".

Published May 2006

For sure, the world's most successful computer program is "hello world". Translations of it are available in almost every programming language and pattern.

"Ajax" is a community acronym used by the software industry for a set of related Web browser technologies, which, in combination deliver the rich client user experience for Web applications. The promise of Ajax is to close the usability gap that currently exist between browser based web applications and existing client-server desktop applications. In this article you'll learn how to translate "hello world" into Ajax, going from simple to more advanced in small steps.

About Ajax

"Ajax" stands for
Asynchronous Javascript and XML and is the heart of the "Web 2.0" paradigm. By one definition (there are several), Web 2.0 is shorthand for a new generation of Web applications that offer increased responsiveness and richness of UI components, moving Web applications closer to what users experience with client-server desktop clients. Ajax is not a software standard but rather describes of a set of technologies—including JavaScript, the Document Object Model (DOM), and the browser XmlHttpRequest object (XmlHttp object in IE)—that are used in combination to build interactive, browser-based user interfaces.

Because of the browser XmlHttpRequest object, Ajax applications can use asynchronous communication to retrieve data from the server. There is no 100 %-clear definition of what makes a Web application "Ajaxian." Therefore, the best way to look at Ajax is as a programming pattern to build the next generation of Web applications that bring us closer to what end users know from their client-server desktop clients. It is important to note that Ajax is not bound or related to J2EE and Java; rather, it can be used with .NET, PHP, CGI, and Perl as well. The good news about Ajax is that it doesn't require an example more complex than "hello world" to explain its principles.

This how-to provides you with a first-class, hands-on experience for building an Ajax type of application in J2EE with Oracle JDeveloper.

The technologies used within the exercises are:

JavaScript - The JavaScript language executes on the browser client and is in the core of Ajax. You use JavaScript to perform logical, mathematical and functional operations on the client side. Using The Document Object Model (DOM), JavaScript can be used to dynamically manipulate a Web page displayed in the browser.

Document Object Model (DOM) - A virtual tree representation of a HTML page document that is kept in the browser memory where it can be accessed using JavaScript. The DOM tree can be used to dynamically manipulate UI components of the current HTML page in the browser.

XmlHttpRequest - The XmlHttpRequest object is a browser-side API to access a remote server and is available in almost all of modern browsers. The XmlHttpRequest object enables a client to fetch new data from a remote server using http
GET or
POST requests without the need to navigate across pages. The server access can be programmed as synchronous or asynchronous, the latter using a JavaScript callback mechanism.

Style Sheets (CSS) - CSS is used to define the look and feel and the positioning and sizing of components on a server. Using external CSS resources, you can keep the visual appearance of an application independent from the representation of the view, which in the Ajax case mostly is built using JavaScript.

HttpServlet - The HttpServlet is used to simulate the server session to demonstrated behind the scene data fetching with Ajax.

Build an Ajax application with Oracle JDeveloper 10.1.3

Following the details listed in this section you build "Hello Ajax" incrementally starting with a static HTML client reading a text string from a server-side file. By the end of this how-to, you will have developed a Web page that reads its data from a servlet on the server, using a CSS style sheet to establish alternating background colors for the retrieved data rows. An input text field allows you to provide your input to the printed message. Download a complete Oracle JDeveloper 10.1.3 workspace
here.

Build a Synchronous Ajax page

1.

Open Oracle JDeveloper 10.1.3 and create a new application, by choosing
File > New > New Application from the JDeveloper menu and New Gallery. In the create dialog, type in the JDeveloper application name as "HelloAjax" and provide other information as needed. You don't need to select an application template because a default JDeveloper project is sufficient.

2.

Give the default JDeveloper project a name like "Ajaxprj".

3.

Open the JDeveloper New Gallery by choosing
New from the context menu on the Ajaxprj node.

4.

Create a new HTML page
helloAjax.hml. Verify the helloAjax.html file is created in the public_html directory so it can be run by the embedded OC4J web container.

5.

Create a text file
helloAjax.txt. To create the text file, choose the
General > Simple Files entry in the JDeveloper New Gallery.

6.

Create a JavaScript file
helloAjax.js.The JavaScript file can be created as an option of the
Web Tier > HTML entry. Again, make sure both file are created in the public_html directory of the JDeveloper "Ajaxprj" project

7.

Open the
helloAjax.html file in the source code view, set the cursor between the <head></head> elements and select
Html Common > Script from the JDeveloper Component Palette (ctrl+shift+P). In the opened dialog, select the
helloAjax.js file entry.This creates a reference to the JavaScript in the html file to ensure the JavaScript code is available when the HTML page is loaded to the client.

8.

Open the
helloAjax.js file in the source editor and copy the following JavaScript code into it.

The JavaScript is a reusable code that initializes the XmlHttprequest object and returns a handle to the caller. The
XmlHttpRequestObject variable is created as a handler to the
createXmlHttpRequest function. JavaScript allows object oriented programming to some degree and using a variable as a reference to a function avoids naming conflicts in case that a JavaScript file contains multiple functions with similar names.

The
XmlHttpRequestObject variable of the
createXmlHttpRequest function is created to hold the XmlHttpRequest object reference. Following the code, you will notice that we look for the XmlHttpRequest object and the XmlHttp object. Though InternetExplorer and the rest of the browser world support XmlHttprequest objects, its named differently in InternetExplorer. If the first condition
typeof XMLHttpRequest != "undefined" succeeds then the client browser is Mozilla, Safaris or any non IE browser that supports asynchronous JavaScript request and response. If the condition
window.ActiveXObject is true then the client browser is IE. The Microsoft browser also distinguishes its version of XmlHttp support and as a developer you should prefer using the latest available support. The
for() loop tests the browser support for a specific XmlHttp version from newer versions to older versions and references the most recent version support in the XmlHttpRequest object reference.

Using the XmlHttpRequest object reference, you can issue server calls from a HTML page using JavaScript. Dependent on whether a request is issued synchronously or asynchronously, the browser waits for the server to respond or uses a JavaScript callback handler to notify the page about the server response. Following this hands-on, you will work with both options.

9.

Save your work.

10.

Open the helloAjax.txt file in JDeveloper and type
hello Ajax as a text message and save the file.

11.

You are done with the pre-requisite work and can now focus on building the HTML page, which is the actual Ajax client. Open the
helloAjax.html file in the source view editor.

12.

Place the cursor between the <head>...</head> elements and select the
Html Common > Script entry in the Component Palette. This time you don't reference an external JavaScript file but type the JavaScript code content directly into the
Content text area of the dialog. The following JavaScript code turns the static HTML file into a dynamic Ajax client, referencing the
xmlHttpRequestHandler created in the helloAjax.js script.

The function
doTheAjaxThing is called from an input button on the HTML page and obtains the "hello Ajax" strings from the server side text file. After pressing
OK on the Script dialog, the function is added to the HTML page header, wrapped in a pair of <script>...</script> elements. The difference between JavaScript code that is referenced from a file and JavaScript directly added on a page is that the former can be reused within many other HTML pages, which is why it should contain generic code only.

The script creates a variable
requestObject that obtains a handle to the XmlHttpRequest object using the
xmlHttpRequestHandler reference in the helloAjax.js file. Note that the page don't need to bother whether it runs in IE or Mozilla. Sorting out the browser type is deferred to the reusable JavaScript file. On the
requestObject handle you call open() passing three arguments. The first arguments specifies the access, which can be any allowed server access like GET, POST or HEAD. The second argument references the URI source to read from. In this example the resource is a text file, later it will be a HttpServlet. Note that JavaScript security, also referred to as the JavaScript sandbox, doesn't allow you to access any other server than the one the code is downloaded from. The third argument is a Boolean type that defines whether the call should be synchronous (false) or asynchronous (true). Using synchronous requests will makes the browser wait for the serve to respond, which is not what you want to see happening in a production application when fetching lot of data from the server. You will use asynchronous server access later in this hands-on.

The
requestObject.send(null)> method call on the XmlHttp Request object sends the request to the server. If this was a POST request, then the
null argument value could be replaced with a list of request parameters that for the server to work with.

Because this example uses a synchronous server call, the browser waits for the server response before proceeding. The http code send by the server on success of the request is 200. The success condition is checked with
requestObject.status==PAGE_SUCCESS before the server response is read from the requestObject using a call to
requestObject.responseText, which is method exposed on the XmlHttpRequest object. The rest of the code references a
DIV HTML element on the page that is accessible through the DOM tree and that is used to display the server response.

Note: The
requestObject.responseText is used to access plain text response from the server. To obtain XML formatted data, use the XMLHttpRequest object's responseXML method instead. For an example of how to use responseXML, see
"A Hype-Free Introduction to Ajax". Applications that need to dynamically determine the response types can do this as follows:

var contentType =
requestObject.getResponseHeader("content-type");

The possible returned content types are 'text/xml' or 'text/plain', indicating XML or plain text content.

13.

The remaining work left is to create a input button on the HTML page that calls the >
doTheAjaxThing function and a
<DIV> element that displays the result. In the code editor view of the helloAjax.html file, copy the following HTML source between the <body>...</body> elements.

The
onclick event that is added to the input button calls the
doTheAjaxThing method on mouse click. The <DIV> element has an
id attribute that is used to give the element a unique name so it can be accessed directly in the DOM tree.

14.

To avoid the browser caching the static HTML page, which can be annoying when testing Ajax pages, add the following line between the <head> and </head> element

<meta http-equiv="pragma" content="no-cache"></meta>

Note that you can have many <meta ...> elements set between the <head> elements. This one tells the browser not to cache this page and its content.

Note that the page appends all strings read from the server. To prove that this ia Ajax, hit the page reload button and see all the string getting deleted from the page.

Congratulation, you created your first Ajax program!

Enhancing Your Ajax application

The following exercise is builds on the previous example and replaces the helloAjax.txt file with a Http Servlet. Because a servlet is more dynamic than a file in that it can contain logic to modify the format of the response based on pre-defined definitions, you use a CSS style sheet to color the returned rows alternating and also display a text typed in by the page user.

1.

Open the
JDeveloper New Gallery using the
New option on the
Ajaxprj project. Choose the
CSS File choice of the
Web Tier > HTML tree entry. Give the file a name of
helloAjax.css, and again make sure it gets added under the
public_html directory.

The
div.message b.red tag writes all <b> elements with a gray background color and a red font color that have a class attribute of "red" and that are embedded as a child element in a HTML <DIV> element with a class attribute of message. The <DIV> and <b> HTML elements are created dynamically as part of the server response returned to the asynchronous Ajax request.

4.

To create the Http Servlet, select
New from the JDeveloper context menu and select the
Web Tier > Servlets entry. From the available items, choose
HTTP Servlet. Name the servlet class
HelloAjax and keep the default values for all other fields and dialogs. This will create a Http Servlet that is mapped to the
/helloajax name in the web.xml deployment descriptor.

5.

Add two instance fields

private int counter = 0;
private String name="";

to the servlet, directly under the
private static final String CONTENT_TYPE = "text/html; charset=windows-1252"; field.

The servlet expects a request parameter "name" that contains the user input to append to the Hello Ajax message. Every even row is printed with a CSS class reference 'green' and every uneven row with a CSS class reference of 'red'. Because the XmlHttpRequest output is printed within a pair of <DIV></DIV> elements, the stylesheet to color the output addresses the <b> element as
div b.green or
div b.red. As you will later add the class="message" attribute to the HTML DIV element, the resulting CSS reference is
div.message b.red and
div.message b.green, which are the CSS names you defined earlier in the helloAjax.css file.

The HTML file now contains an additional text field for the user to type in a string for output. The name input field has an
id attribute of "name", which makes it accessible from JavaScript using the browser DOM tree. The exact location of the text field is
document.form1.name.

The last changes is the
class attribute, which is added to the
<div> element to be addressed by the style sheet file.

10.

Switch to the design view of the helloAjax.html file. Select the
helloAjax.css entry in the Application Navigator and drag and drop it onto the HTML page. This creates a reference from the HTML page to the stylesheet at runtime.

11.

Again, open the helloAjax.html file in the source code view and look for the
doTheAjaxThing() method between the <head></head> element pair.

12.

Add the first line highlighted in bold to the JavaScript function and replace the existing
requestObject.open() method call with the second highlighted code line.

The first line creates a variable param that reads the value of the input text field. To make sure the value isn't left empty, JavaScript is used to test the length of the input string and if it is left blank, replaces it with "nobody".

The
requestObject.open method call is changed in that instead of the helloajax.txt file, it now refers to
helloajax, which is the server side web.xml name of the Http Servlet. Because the request is a GET request, the
name request parameter is appended to the servlet reference using a question mark '
?'. As you see, from a XmlHttpRequest point of view, the change required to switch between calling a static file on the server and calling a servlet instead is minor.

13.

Optionally, change the HTML <tile> element in the header to

<title>Hello Ajax from servlet</title>

14.

Save your work and run the
helloAjax.html file.

You will notice a slight delay the first time you run the application. This delay is caused by the initial startup time required by the servlet. This delay doesn't make a good impression to the users, which is why in the next section you are going to change the synchronous XmlHttpRequest call to an asynchronous call.

Making Your Ajax Application Asynchronous

Asynchronous requests allow the application user to continue his work in the browser while waiting for the server response. If the user cannot proceed with his work unless the server responded, then still using an asynchronous request is a better approach than synchronous call. For example, instead of the user staring at a pressed button, you can show him some nice "end-user entertainment" widgets, like a progress bar. For this part of the hands-on we are not going that far and instead only change the synchronous call to a asynchronous call.

1.

Replace the existing JavaScript section that the defines the
doTheAjaxThing() function in the helloAjax.html page with the following to make the request asynchronous:

The differences between this asynchronous request handling and the previous synchronous call are highlighted in bold. Because the browser doesn't wait for the server to respond, this example uses a JavaScript callback that is a function called by the XMLHttpRequest object on state change. There are four states of interest:

0

The request is initialized, which is the state before the open() method is called on the XmlHttRequestObject.

1

The request is ready to be sent.

2

The request was sent and is processed on the server.

3

The server processes the request

4

The response is complete and available for further client side handling

The state you are most interested in is indicated by a state value of 4. In the JavaScript function above, for better reading, this value is assigned to a variable JavaScript variable READY_STATE_COMPLETE. Don't confuse state with status. The state is returned by the XmlHttpRequest, while the status is a HTTP code that tells you about the requested access. If for example the requestObject status is 403 then you accessed a server resource that you are not authenticated or authorized for. This status for sure needs a different client handling than 200, which indicates a successful server access.

To register the JavaScript callback handler
onReadyStateChangeResponse with the XmlHttprequest object, you assign it as a value to the XmlHttpRequest onreadystate method,
requestObject.onreadystatechange=onReadyStateChangeResponse, which is one of the exposed functionality of the XmlHttpRequest object.

The JavaScript function
onReadyStateChangeResponse() can be named as you please, as long as the same name is assigned as a value to to
requestObject.onreadystatechange. Before you can read the returned message body you need to perform the mentioned checks for the ready state to be 4 and the request status to be 200. The rest of the code is the same as in the examples before.

2.

Save your work and run the application.

Note that still you see a delay between pressing the button and the server response, but this time the button is released immediately.

Basic Troubleshooting

The most common problems you find with Javascript is the misspelling of components and variables and the addressing of non-existing properties on a DOM component. There exist debuggers for JavaScript that you can get for free on the Internet. However, for "quick and dirty" debugging, you can use alert("test"); or document.write("test"); statements to print information on the client. On the server side you use the Oracle JDeveloper debugger for the Java code you access, or use System.out.println("test"); there as well.

Summary

This how-to aims to give you a first hands-on experience of how to work with Ajax in Oracle JDeveloper 10.1.3. As I mentioned in the introduction, there is not much required to program with Ajax. If you want to do more advanced Ajax programming, you will find yourself looking deeper into JavaScript, CSS, and http message codes. From the perspective of the XmlHttpRequest object, this is it: A simple object (or API) with a big impact on Web application development. Because no specific Ajax support is needed by an IDE, you can browse the Internet for more examples and existing javaScript libraries that you use within your application development using Oracle JDeveloper.

Although the technology involved in building Ajax applications has been around for years, Ajax as a pattern is new and a lot of growth is expected in this area. It should lead to more improved clients that eventually will become mainstream for all kinds of Web application developers, including those with a 4GL background.

Read More about Oracle and Ajax

Read and learn more about Oracle and Ajax! Visit our
Ajax page on the Oracle Technology Network (OTN) website.